1. Field of the Invention
The present invention pertains to a method and apparatus for adjusting the tension in an array of chains, for example, in a drill string compensator, so as to provide uniform load distribution to all chains of the array.
2. Description of the Prior Art
There are many instances when arrays of chains are used to support and/or transmit loads. It is preferable to have the chains of the array share the load equally to thereby increase the safe operating margin and working life for the chains. However, the means and methods for tensioning chains used to date have been rather elementary and have not produced any real degree of uniformity in tensioning of the chains. For example, one method calls for striking the chains witha mallet and attempting to gauge tension by the timbre and resonance of the resulting sound. An equally ineffective method involves pushing against the chains by hand, much in the same manner as a motorist might do to check the various belts under the hood of his car. It will be readily appreciated that neither of these terminals would be likely to produce any degree of uniformity in tensioning nor would they be likely to work in noisy environments or with large, heavy chains.
An example of the use of chain arrays can be found in drill string compensators which are devices to compensate for the vertical motion imparted to marine vessels by normal wave action and are necessary when the vessel is supporting submerged equipment as, for example, during drilling or other undersea operations. Such vessels usually have a derrick fixed to the deck of the vessel with a crown block at the upper end of the derrick and a traveling block suspended from the crown block by a line arrayed between the sheaves of the blocks. The traveling block has a hook from which an elevator or the like is connected to depend into the water to the ocean floor. A fluid actuated system provides a reciprocal motion to allow the hook to be maintained at a fixed location relative to the undersea floor as the traveling block heaves with the wave action imparted to the vessel. Such compensators require the weight of the compensation apparatus to be supported by the same line by which the traveling block is suspended from the crown block which is fixed to the mast of the derrick.
Motion compensators are known for keeping either the crown block or the traveling block stationary relative to the undersea floor as the floating vessel moves. In either case, the respective block is supported by means of a pair of direct acting hydraulic or pneumatic piston and cylinder assemblies with multiple chain arrays providing the actual interconnect between the main frame and movable frame of the compensator device. Heretofore, the chains have been adjusted for tension by tightening a nut and bolt arrangement at one end of each chain with the relative tension between the chains being determined by either of the two methods discussed above, namely pushing on the chains or by hitting the chains with a mallet and attempting to ascertain like tones between the various chains of the array.
It will be readily appreciated by those skilled in the art that in a drill string compensating system the size and weight of the chains and their associated assemblies are such that manually pushing against the chains would hardly yield any adequate information as to the tension in each chain or comparison of tension between two chains. It is also clear to those skilled in the art that the background noise on a floating drill rig would be such that it would be substantially impossible to ascertain close or like tones, even assuming that the roughneck had sufficient musical talent to ascertain and compare such tones.